How to write the elements $Se$, $Rb$, $S$ in decreasing atomic size?

Hint: The atomic size of elements depend on the electronegative character of the element, meaning, if an element is highly electronegative the size of that element would shrink as the outermost electrons present in the shells would experience higher effective nuclear charge from the nucleus.

Complete step-by-step answer:
The atomic size or the atomic radius is dependent on various factors like the shells of electrons, shielding of the electrons by orbitals, and effective nuclear charge. The nucleus is surrounded by a number or circular orbitals which are termed as energy shells. With the increase of the value of the principal quantum number of an element, the atomic radius of that element also increases as we go down the group.

The electrons and the protons which are present in an element have opposite charges hence they tend to attract each other. This attraction is called the nuclear charge. So, as we move across a period, with every consecutive element, one electron gets added to its outermost shell, along with an increase in nuclear charge as a proton is also being added in the nucleus. And with increase in nuclear charge, the atomic size decreases as the electrons present in the shells get attracted to the nucleus even more and so the radius decreases.

If we consider the three elements which are provided to us, the most electronegative is $Rb$ followed by $Se$ and then $S$. Now because of the higher electronegativity of the element $Rb$, the outer electrons would experience greater effective nuclear charge, hence it will get pulled towards the nucleus more than the other two elements, so the size of the element would decrease.

Hence according to the trend of electronegativity, the decreasing order of atomic size would become $Rb$, $Se$, $S$.

Note:
i) The atomic size of the elements decreases as we move across a period because with each consecutive element, one electron and one proton is added to the element resulting in greater nuclear charge experienced by the electrons, hence smaller size.

ii) However, when we move down the group in a periodic table the atomic size increases as, with every consecutive element a new shell gets added to the element and so the effective nuclear charge experienced by the outermost electrons is less, hence bigger size.